A number of organic or inorganic substances have been recommended as photoconductive materials for electrophotograhic photoreceptors and the like. Known organic photoconductive materials include bisazo pigments, trisazo pigments, phthalocyanine pigments, cyanine compounds, and pyrylium compounds. It has recently been reported that certain kinds of squarylium compounds exhibit excellent photoconductive characteristics.
Known squarylium compounds include symmetrical comopunds in which the right and left of the 4-membered ring thereof are symmetrical with each other and asymmetrical compounds in which the right and left of the 4-membered ring thereof are asymmetrical. These compounds are inclusively represented by formula (III): ##STR1## wherein R.sub.1, R.sub.2, R.sub.5, and R.sub.6 each represents a substituted or unsubstituted alkyl group, a substituted or unsubstituted phenyl group, or a substituted or unsubstituted benzyl group; and R.sub.3, R.sub.4, R.sub.7, and R.sub.8 each represents a hydrogen atom, a substituted or unsubstituted alkyl group, a halogen atom, a hydroxyl group, a carboxyl group, a carbonamido group substituted with a substituted or unsubstituted alkyl group or a substituted or unsubstituted phenyl group, or a sulfonamido group substituted with a substituted or unsubstituted alkyl group or a substituted or unsubstituted phenyl group.
In general, the symmetrical squarylium compounds are synthesized by reacting squarylic acid with the corresponding aniline derivative in an alcohol.
On the other hand, the asymmetrical squarylium compounds are synthesized through the following reactions. Squarylic acid is chlorinated with a chlorinating agent, e.g., thionyl chloride, and the chlorinated compound is reacted with an aniline derivative to form a chlorocyclobutenedione derivative. The resulting compound is hydrolyzed to obtain a hydroxycyclobutene derivative, which is then reacted with an aniline derivative different from that used above to synthesize an asymmetrical squarylium compound.
Improved processes for synthesizing squarylium compounds which have been proposed in the past include a process comprising reacting squarylic acid with a compound selected from the group consisting of an aromatic aniline, a phenol, and a phenolsquarylium compound as disclosed in JP-A-60-174750 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a process comprising reacting a half ester of squarylic acid with an aniline derivative in the presence of an acid catalyst as disclosed in JP-A-60-202849, a process comprising reacting squarylic acid with an aromatic aniline in the presence of an aliphatic amine as disclosed in JP-A-60-208361, and a process comprising reacting squarylic acid, an aromatic aniline, and fluoroaniline as disclosed in JP-A-61-87647.
These conventionally proposed processes are characterized in that the reaction conditions are set so as to reduce impurities or, in the alternative, various impurities are intentionally introduced into the synthesis system, in order to improve the electrophotographic characteristics of the resulting squarylium compound for use as a photoconductive material of electrophotographic photoreceptors. However, none of the above-cited references has elucidated what factors influence the electrophotographic characteristics of the produced squarylium compound. Therefore, only a slight alteration in the reaction conditions would result in variation of the electrophotographic characteristics of the resulting squarylium compound when used as a photoconductive material of electrophotographic photoreceptors. In such situations, it has been difficult to continually and reproducibly prepare a stable squarylium compound which is suited for use in electrophotographic photoreceptors processing satisfactory electrophotographic characteristics.